Transcript Transmission Lines

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Conductor Material
• Utility companies use different conductor materials for different
applications.
• Copper, aluminum, and steel are the primary types of conductor
materials used in electrical power systems.
• Other types of conductors, such as silver and gold, are actually
better conductors of electricity; however, cost prohibits wide use of
these materials.
Copper
• Copper is an excellent conductor and is very popular. Copper is
very durable and is not affected significantly by weather.
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Aluminum
• Aluminum is a good conductor but not as good or as durable as
copper.
• However, aluminum costs less. Aluminum is rust resistant and
weighs much less than copper.
Steel
• Steel is a poor conductor when compared to copper and
aluminum; however, it is very strong.
• Steel strands are often used as the core in aluminum conductors
to increase the tensile strength of the conductor.
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Conductor Types
• Power line conductors are either solid or stranded.
• Rigid conductors such as hollow aluminum tubes are used as
conductors in substations because of the added strength against sag
in low-profile substations when the conductor is only supported at
both ends.
• Rigid copper bus bars are commonly used in low-voltage switch
gear because of their high current rating and relatively short lengths.
The most common power line conductor types are shown below:
Solid
• Solid conductors (Figure 3-2) are typically smaller and stronger
than stranded conductors. Solid conductors are usually more
difficult to bend and are easily damaged.
Bus Bar
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Stranded
• Stranded conductors have three or more strands of conductor
material twisted together to form a single conductor.
• Stranded conductors can carry high currents and are usually
more flexible than solid conductors.
Aluminum Conductor, Steel-Reinforced (ACSR)
• To add strength to aluminum conductors.
• Figure shows steel strands that are used as the core of aluminum
stranded conductors. These high-strength conductors are normally
used on long span distances, for minimum sag applications.
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Fig.1 Solid conductor
Fig.2 Stranded conductor
Fig.3 ACSR conductor
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American Standard Wire Gauge (AWG)
• The American Standard Wire Gauge is an old standard that is
used for relatively small conductor sizes.
• The scale is in reverse order i.e. the numbers get smaller as the
conductors get larger. The circular mils standard of measurement is
used for large conductor sizes.
Circular Mils
• Conductors greater than AWG 4/0 are measured in circular mils
(cmills). One circular mil is equal to the area of a circle having a
0.001 inch (1 mil) diameter.
• For example, the magnified conductor in the Figure 3-5 has 55
circular mils.
• Conductors sized in circular mils are usually stated in thousands
of circular mils (i.e., kcm).
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Table 3-1 shows typical conductor sizes and associated current
ratings for outdoor bare ACSR conductors having a current rating of
75°C rise above ambient. The table also shows the equivalent
copper size conductor.
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Insulation and Outer Covers
• Metal wire current-carrying conductors can be insulated or noninsulated when in use.
• Normally, there are two types of insulation in electrical power
transmission.
1. Insulators are used by means for separating the bare wires from
the grounded structures.
2. Other type of insulator are used as outer cover for conductor.
High-voltage insulated conductors are normally used in
underground systems. Insulated low-voltage service wires are
also used for residential overhead and underground lines.
• Insulator use plastic, rubber, or other jacketing materials for
electrical isolation.
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Voltage Classes
• Table 3-2 shows the various transmission and subtransmission
system voltages.
• It is quite common to use subtransmission voltages to transport
power over medium distances (i.e., across large populated areas) or
to transport power over long distances if the total current
requirement is low, such as for serving less populated areas that are
far away.
• The higher transmission system voltages tend to be more
standardized compared to the lower distribution voltages.
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Voltage Class is the term often used by equipment manufacturers
and power companies to identify the voltage that the equipment will
be connected.
• A manufacturer might use the voltage class to identify the
intended system operating voltage for their equipment.
• For example, a circuit breaker might be a 125 kV voltage class
piece of equipment that is operating at a nominal 115 kV voltage.
Voltage Category is often used to identify a group of voltage
classes. For example, “extra high voltage” (or EHV) is a term used
to state whether an equipment manufacturer builds transmission
equipment or distribution equipment, which would be categorized
as “high-voltage equipment” (or HV).
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Underground Transmission
• Underground transmission is usually three to ten times
more costly than overhead transmission due to right of
way requirements, obstacles, and material costs.
• It is normally used in urban areas or near airports
where overhead transmission is not an option.
• Cables are made of solid dielectric polyethylene
materials and can have ratings on the order of 400 kV.
Figure 3-6 shows a 230 kV underground transmission line.
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Dc Transmission Systems
• dc Transmission systems are sometimes used for
economic reasons, system synchronization benefits, and
power flow control.
• The three-phase ac transmission line is converted into
a two-pole (plus and minus) dc transmission line using
bidirectional rectification converter stations at both ends
of the dc line.
• The converter stations convert the ac power into dc
power and vice versa. The reconstructed ac power must be
filtered for improved power quality performance before
being connected to the ac system.
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Dc Transmission Systems…….
• dc transmission lines do not have phases; instead, they
have positive and negative poles.
• There are no synchronization issues with dc lines. The
frequency of dc transmission is zero and, therefore, there
are no concerns about variations in frequency between
interconnected systems.
• A 60 hertz system can be connected to a 50 hertz
system using a dc line. For economic reasons, the dc line
may have advantages over the ac line in that the dc lines
have only two conductors versus three conductors in ac
lines.
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Dc Transmission Systems…….
• For economic reasons, the dc line may have
advantages over the ac line in that the dc lines have only
two conductors versus three conductors in ac lines.
• The overall cost to build and operate a dc line,
including converter stations, may cost less than an
equivalent ac line due to the savings from one less
conductor, narrower right of ways, and less expensive
towers.